In recent years, the development of mobile digital radio communication systems by which not only control signals but also digitized communication data such as communication voice signals may be transmitted has advanced.
Known digital radio communication devices used in this type of system include devices provided with an equalizer in the receiving system. This equalizer is a functional circuit that compensates for intersymbol interference produced at times when there is delayed propagation of radio signals due to multipath fading, and is constructed using a transversal filter, for example.
With conventional equalizer-equipped digital radio communication devices of this type, the demodulated signal bit error rate sometimes deteriorates depending on the delay of the delayed wave.
For example, under conditions of reception of two-wave relay waves, digitally demodulated signal bit error rates vary with the amount of delay of the delayed waves in different ways in the manner depicted in FIG. 11, depending on whether the digitally demodulated signals that have been demodulated by the digital demodulation circuit are passed through an equalizer or not.
In the same drawing, the solid line indicates bit error rate characteristics of digitally demodulated signals demodulated in the digital demodulation circuit versus the amount of delay of the delayed waves, and the dashed line indicates bit error rate characteristics for signals produced by equalization in an equalizer of digitally demodulated signals demodulated in the digital demodulation circuit versus the amount of delay of delayed waves.
The same figure indicates that under conditions of a relatively large delay, a delayed wave delay of 0.2 symbols or above, the bit error rate is lower for signals passing through the equalizer. However, under conditions of a delay of less than 0.2 symbols, the demodulated signal error bit rate is lower for those not passing through the equalizer.
Therefore, conventional devices of this type designed so that digitally demodulated signals are always processed through an equalizer are susceptible to deterioration in bit error rate under conditions of a small delayed wave delay, thus lowering reception sensitivity.
In this way, conventional equalizer-equipped digital radio communication devices always process digitally demodulated signals through the equalizer, so that equalization processing is invariably performed even under conditions of an extremely small delayed wave delay such as that during reception of two-way relay waves, when better digitally demodulated signal bit error rates would be sustained by not passing them through the equalizer, and thus have the drawback that the digitally demodulated signal bit error rate is conversely increased.
An object of the present invention is to provide a digital radio communication device that does not perform equalization processing under conditions of extremely small delayed wave delay when better digitally demodulated signal bit error rates would be sustained by not passing them through the equalizer, and that outputs the unmodified output of the digital demodulation circuit as demodulated data so that good bit error rates can always be maintained regardless of delay wave delay, thus contributing to stabilized reception.
Another object of the present invention is to provide a digital radio communication device that achieves, in a simpler circuit structure and even at the expense of a certain amount of bit error rate suppressing effects, a control function whereby digitally demodulated signal bit error rates are always good regardless of delayed wave delay.
A further object of the present invention is to provide a digital radio communication device that can stably sustain a control function whereby digitally demodulated signal bit error rates are always good regardless of delayed wave delay, even cases when travel speed of the device itself changes.